ALBERT

Description: The capillary fringe is a subsurface terrestrial-aquatic interface which can be a significant hotspot for biogeochemical cycling of terrestrially derived organic matter and nutrients. However, pathways of nitrogen (N) cycling within this environment are poorly understood, and observations of temporally discrete changes in nitrate concentrations lack the necessary resolution to partition between biotic or abiotic mechanisms. Here we take an experimental and mechanistic modeling approach to characterize the annual decline of nitrate (NO3−) within floodplain sediments at Rifle, Colorado. At discrete sampling points during 2014 we measured NO3−, ammonia (NH4+), gaseous nitrous oxide (N2O) and the corresponding isotopic composition of NO3−. Coincident with an annual spring/summer excursion in groundwater elevation driven by snowmelt, we observed a rapid decline in NO3− concentrations at three depths (2, 2.5 and 3m) below the ground surface. Isotopic measurements (i.e., δ15N and δ18O of NO3−) suggest an immediate onset of biological N loss at 2m. At 2.5 and 3m, NO3− concentrations declined initially with no observable isotopic response, indicating an initial dilution of NO3− within the well. Following extended saturation by groundwater at these depths we observed subsequent nitrate reduction. A simple Rayleigh model suggests depth-dependent variability in the importance of actively fractionating mechanisms (i.e., nitrate reduction) relative to non-fractionating mechanisms (mixing and dilution). Nitrate reduction was calculated to be responsible for 64% of the NO3− decline at 2m, 28% at 2.5 and 47% at 3m, respectively. Furthermore, we observed the highest concentrations of N2O as groundwater saturated the 2 and 2.5m depth, concomitant with enrichment of the δ15NNO3 and δ18ONO3. A mechanistic microbial model representing the diverse physiology of nitrifiers, facultative aerobes (including denitrifiers), and anammox bacteria indicates that the bulk of biological N loss within the capillary fringe is attributable to denitrifying heterotrophs. However, this relationship is dependent on the coupling between aerobic and anaerobic microbial guilds at the oxic-anoxic interface. Modeling insights also suggest that anammox might play a more prominent role in N loss under conditions where organic matter concentrations are low and rapidly depleted by aerobic heterotrophs prior to the rise of the water table.

Description: Nitrite (NO2-) is a key intermediate in the marine nitrogen (N) cycle and a substrate in nitrification, which produces nitrate (NO3-), as well as water column N loss processes denitrification and anammox. In models of the marine N cycle, NO2- is often not considered as a separate state variable, since NO3- occurs in much higher concentrations in the ocean. In oxygen deficient zones (ODZs), however, NO2- represents a substantial fraction of the bioavailable N, and modeling its production and consumption is important to understand the N cycle processes occurring there, especially those where bioavailable N is lost from or retained within the water column. Improving N cycle models by including NO2- is important in order to better quantify N cycling rates in ODZs, particularly N loss rates. Here we present the expansion of a global 3-D inverse N cycle model to include NO2- as a reactive intermediate as well as the processes that produce and consume NO2- in marine ODZs. NO2- accumulation in ODZs is accurately represented by the model involving NO3- reduction, NO2- reduction, NO2- oxidation, and anammox. We model both 14N and 15N and use a compilation of oceanographic measurements of NO3- and NO2- concentrations and isotopes to place a better constraint on the N cycle processes occurring. The model is optimized using a range of isotope effects for denitrification and NO2- oxidation, and we find that the larger (more negative) inverse isotope effects for NO2- oxidation, along with relatively high rates of NO2-, oxidation give a better simulation of NO3- and NO2- concentrations and isotopes in marine ODZs.

Description: Nitrite (NO2−) is a key intermediate in the marine nitrogen (N) cycle and a substrate in nitrification, which produces nitrate (NO3−), as well as water column N loss processes, denitrification and anammox. In models of the marine N cycle, NO2− is often not considered as a separate state variable, since NO3− occurs in much higher concentrations in the ocean. In oxygen deficient zones (ODZs), however, NO2− represents a substantial fraction of the bioavailable N, and modeling its production and consumption is important to understanding the N cycle processes occurring there, especially those where bioavailable N is lost from or retained within the water column. Here we present the expansion of a global 3D inverse N cycle model to include NO2− as a reactive intermediate as well as the processes that produce and consume NO2− in marine ODZs. NO2− accumulation in ODZs is accurately represented by the model involving NO3− reduction, NO2− reduction, NO2− oxidation, and anammox. We model both 14N and 15N and use a compilation of oceanographic measurements of NO3− and NO2− concentrations and isotopes to place a better constraint on the N cycle processes occurring. The model is optimized using a range of isotope effects for denitrification and NO2− oxidation, and we find that the larger (more negative) inverse isotope effects for NO2− oxidation along with relatively high rates of NO2− oxidation give a better simulation of NO3− and NO2− concentrations and isotopes in marine ODZs.